The origin of the interfacial perpendicular magnetic anisotropy (PMA) induced in the ultrathin Fe layer on the Au(111) surface was examined using synchrotron-radiation-based M\"ossbauer spectroscopy (MS), x-ray magnetic circular dichroism (XMCD), and angle-resolved photoemission spectroscopy (ARPES). To probe the detailed interfacial electronic structure of orbital hybridization between the Fe $3d$ and Au $6p$ bands, we detected the interfacial proximity effect, which modulates the valence-band electronic structure of Fe, resulting in PMA. MS and XMCD measurements were used to detect the interfacial magnetic structure and anisotropy in orbital magnetic moments, respectively. In situ ARPES also confirms the initial growth of Fe on large spin-orbit coupled surface Shockley states under Au(111) modulated electronic states in the vicinity of the Fermi level. This suggests that PMA in the Fe/Au(111) interface originates from the cooperation effects among the spin, orbital magnetic moments in Fe, and large spin-orbit coupling in Au. These findings pave the way to develop interfacial PMA using $p\text{\ensuremath{-}}d$ hybridization with a large spin-orbit interaction.

Perpendicular magnetic anisotropy at the Fe/Au(111) interface studied by Mössbauer, x-ray absorption, and photoemission spectroscopies

We theoretically investigate the interfacial magnetoelectric effect in the BaTiO3/Ni heterostructure. We find that magnetoelectric coupling, specifically, the dependence of the magnetic moment of Ni on the polarization direction, primarily stems from the flow of the screening charge from Ni into BaTiO3 with a minor contribution coming from interfacial chemical bonding. The estimated change in the magnetic moment from screening with respect to bulk Ni is as high as ∼7%, which is a significant modulation. We also examine the effects of interfacial oxidation on the electronic structures and the strength of magnetoelectric coupling in the BaTiO3/NiO/Ni structure. We find an enhancement of the interfacial bonding contribution, making it unfavorable for interfacial magnetoelectric modulation. In addition, we consider the effect of the Ni magnetic domain wall on interfacial coupling and find it to be negligible to the interfacial magnetoelectric effect. 

https://aip.scitation.org/doi/10.1063/5.0079880

Extrinsic magnetoelectric effect at the BaTiO3/Ni interface

The Fe(CoB)/MgO interface is vital to spintronics as it exhibits the tunneling magnetoresistance (TMR) effect and interfacial perpendicular magnetic anisotropy (PMA) simultaneously. To further enhance TMR and PMA for the development of high-density magnetoresistive random access memory, it is essential to clarify the behavior of Fe atoms interfaced with MgO. This study reveals that the spin and orbital magnetic moments of Fe are enhanced at the Fe/MgO interface. The enhancement in the orbital magnetic moment is much more significant than that predicted by the standard density functional theory. Theoretical calculations based on the orbital polarization correction reproduce this enhancement, the origin of which is attributed to the electron–electron correlation resulting from electron localization at the Fe/MgO interface. The present findings highlight the importance of electron–electron correlation at ferromagnet/oxide interfaces, which has often been disregarded in spintronics. 

Electron Correlation Enhances Orbital Polarization at a Ferromagnetic Metal/Insulator Interface: Depth-Resolved X-ray Magnetic Circular Dichroism and First-Principles Study

Abstract To realize future spintronic applications with magnetic skyrmions—topologically nontrivial swirling spin structures—it is essential to achieve efficient writing and deleting capabilities of these quasi-particles. Electric-field assisted nucleation and annihilation is a promising route, however, the understanding of the underlying microscopic mechanisms is still limited. Here, we show how the stability of individual magnetic skyrmions in an ultrathin transition-metal film can be controlled via external electric fields. We demonstrate based on density functional theory that it is important to consider the changes of all interactions with electric field, i.e., the pair-wise exchange, the Dzyaloshinskii–Moriya interaction, the magnetocrystalline anisotropy energy, and the higher-order exchange interactions. The energy barriers for electric-field assisted skyrmion writing and deleting obtained via atomistic spin simulations vary by up to a factor of three more than the variations of the interactions calculated from first-principles. This surprising effect originates from the electric-field dependent size of metastable skyrmions at a fixed magnetic field. The large changes in lifetimes allow the possibility of electric-field assisted thermally activated writing and deleting of skyrmions. 

/pdf/electric-field-driven-stability-control-of-skyrmions-in-an-3uan9uub.pdf

Electric-field driven stability control of skyrmions in an ultrathin transition-metal film

Using density functional theory (DFT), we study how the stability of individual magnetic skyrmions in an ultrathin transition-metal film can be controlled via the external electric fields. For applied electric fields of $\mathcal{E}$= $\pm 0.5$ V/A, we find changes from 8 to 30$\%$ of the Heisenberg exchange, the Dzyaloshinskii-Moriya interaction, the magnetocrystalline anisotropy energy, and the higher-order exchange interactions. Based on atomistic spin simulations using the DFT parameters, we find that the energy barriers for electric field assisted skyrmion writing and deleting can vary by up to a factor of three more than the variations of the interactions. This unexpected result originates from the electric field induced shifts of the critical magnetic field, marking the onset of the field-polarized phase, which exhibits metastable skyrmions. The shift leads to an electric field dependent change of the skyrmion radius at a fixed magnetic field and explains the enhanced energy barrier variations.

Electric field driven stability control of skyrmions in an ultrathin transition-metal film

The surface magnetism of Fe(001) was studied in an atomic layer-by-layer fashion by using the in situ iron-57 probe layer method with a synchrotron Mossbauer source. The observed internal hyperfine field H_{int} exhibits a marked decrease at the surface and an oscillatory behavior with increasing depth in the individual upper four layers below the surface. The calculated layer-depth dependencies of the effective hyperfine field |H_{eff}|, isomer shift δ, and quadrupole shift 2ϵ agree well with the observed experimental parameters. These results provide the first experimental evidence for the magnetic Friedel oscillations, which penetrate several layers from the Fe(001) surface.

S. Li

Papers

Magnetic Friedel Oscillation at the Fe(001) Surface: Direct Observation by Atomic-Layer-Resolved Synchrotron Radiation ^{57}Fe Mössbauer Spectroscopy.